JP-3505: Update ASN rules for NRS MOS compromise dither patterns

[hbushouse]

Resolves JP-3505

Closes #8183

This PR updates the level-2 ASN rules to accommodate NIRSpec MOS dither patterns that include sub-pixel dithering, so that only exposures from other nod positions get designated as "background" members in spec2 ASN files. This mechanism was already in place for NIRSpec fixed-slit (FS) observations that use both primary nods and sub-pixel dithers, so it just needed to be extended to also include MSA observations.

Checklist for maintainers

• added entry in CHANGES.rst within the relevant release section
• updated or added relevant tests
• updated relevant documentation
• added relevant milestone
• added relevant label(s)
• ran regression tests, post a link to the Jenkins job below.
  How to run regression tests on a PR
• Make sure the JIRA ticket is resolved properly

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (19925fc) 77.37% compared to head (f1780b0) 75.26%.

Files	Patch %	Lines
jwst/associations/lib/rules_level2_base.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8184      +/-   ##
==========================================
- Coverage   77.37%   75.26%   -2.11%     
==========================================
  Files         451      470      +19     
  Lines       35289    38430    +3141     
==========================================
+ Hits        27304    28924    +1620     
- Misses       7985     9506    +1521     

Flag	Coverage Δ		*Carryforward flag
nightly	77.37% <ø> (-0.01%)	⬇️	Carriedforward from c1829ec

*This pull request uses carry forward flags. Click here to find out more.

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[hbushouse]

Regression test run started at https://plwishmaster.stsci.edu:8081/job/RT/job/JWST-Developers-Pull-Requests/1135

[hbushouse]

1 regression test failure was due to an unrelated unclosed file error. Succeeds when run locally on my PR branch.

[hbushouse]

The unit test that I added uses an example pool file taken directly from one observation in program 2640, consisting of an exposure sequence with 3 primary nods, 3 subpixel dithers at each nod, and using both NRS detectors. So a total of 18 DMS exposures, where each "spec2" asn file that gets created has one of the 18 exposures treated as "science", and the 6 exposures from other nod positions (2 nods x 3 subpix dithers) as "background" members.